The present invention relates to an optical access system or optical subscriber communication or optical CATV employing a high density wavelength-multiplexing system.
In FIG. 26 there is shown an example which employs a wavelength multiplexing system in an access network from the central office to subscribers. Let it be assumed that wavelengths .lambda..sub.1 to .lambda..sub.4 (the number of subscribers in this example is four) are assigned to subscribers 301 to 304, respectively. Optical signals of wavelengths .lambda..sub.1 to .lambda..sub.4 from optical transmitters 311 to 314 of the central office are multiplexed by an optical coupler-splitter 315 and then forwarded to one optical fiber 316. The multiplexed optical signals are split, by an optical coupler-splitter 318 in a node 317 placed near the subscribers, into optical signals of wavelength .lambda..sub.1 to .lambda..sub.4 respectively corresponding to subscribers 301 to 304, thereafter being received by optical receivers 321 to 324 of the subscribers 301 to 304 via optical fibers 331 to 334. Such a network is commonly called a passive double star (PDS) network. As regards a message from the subscriber, for example, 301 to the central office, an optical signal of the wavelength .lambda..sub.1 sent out from an optical transmitter 325 is transmitted over an optical fiber 341 to the node 317, where it is wavelength-multiplexed by an optical coupler-splitter 319 with optical signals from other subscribers 302 to 304 transmitted over optical fibers 342 to 344, and the thus wavelength-multiplexed optical signals are transmitted to the central office over one optical fiber 329. In the central office, the multiplexed optical signals are split by an optical coupler-splitter 330 into signals of the respective wavelengths, which are fed to individual optical receivers 351 to 354.
In the optical access system employing the wavelength-multiplexing scheme, the wavelengths are usually spaced 1 to 2 nm apart. The oscillation wavelength of a semiconductor laser as a light source undergoes a temperature change of 0.1 nm/.degree.C. even if it is a distributed feedback laser. If the temperature of the optical transmitter placed in the subscriber's home varies 20.degree. C., the oscillation wavelength will change by a value of 2 nm. When the wavelength of the optical transmitter 325 of the subscriber 301, initially set at a wavelength .lambda..sub.1 as shown by (a) in FIG. 27, changes to a wavelength .lambda..sub.1 ' due to a change in the ambient temperature of the optical transmitter 325, a crosstalk to adjacent channels will occur even if the wavelength characteristics of the optical coupler-splitter 319 placed in the node 317 of the network and the optical coupler-splitter 330 of the central office do not vary as shown by (b) and (c) in FIG. 27. To avoid this, it is necessary to stabilize the wavelength of the optical transmitter placed in the subscriber's station. In many cases, a Peltier element is used for temperature control of the light source in a quest to stabilize its wavelength.
The optical coupler-splitter 318 (or 319) of the node 317 is mounted in a conduit line or on a mast, and it is considered that freedom from maintenance is a precondition for the design of an economical system. Even if the wavelength of the optical transmitter 325 of the subscriber 301 is stabilized at a wavelength .lambda..sub.1 as depicted by (a) in FIG. 28, the wavelength characteristic of the optical coupler-splitter 319 in the node 317 may sometimes shift by a value .DELTA..lambda. due to adverse environmental conditions as shown by (b) in FIG. 28. Even if the optical coupler-splitter is formed of quartz glass, its wavelength characteristic undergoes a temperature change of 0.01 nm/.degree.C. owing to the temperaturedependency of the refractive index of quartz glass. A 100.degree. C. temperature change (for example, an operating temperature of -40 to 65.degree. C. is required outdoors) will cause a wavelength change of 1 nm . That is, .DELTA..lambda.=1 nm. This influence is serious in the high density wavelength-multiplexing system. Provided that the wavelength characteristic of the optical coupler-splitter at the central office side is such as shown by (c) in FIG. 28, the optical signal (a) of the wavelength .lambda..sub.1 from the optical transmitter 325 of the subscriber 301 is intercepted owing to the deviation characteristic (b), and hence it does not reach the central office. Even if the optical signal is allowed to pass through the optical coupler-splitter 319 of the node 317 by changing the wavelength of the light source of the optical transmitter 325 to .lambda..sub.1 +.DELTA..lambda., the optical signal is inhibited from the passage through the optical coupler-splitter 330 at the central office side, and hence it does not reach the optical receiver 351 of the central office.
In an optical network in which the wavelength characteristics of optical coupler-splitters and optical filters vary due to an ambient temperature change, no proposals have been made so far on a light source wavelength control method and on a wavelength-multiplexing optical access system utilizing the method.
In ordinary optical access systems using the passive double star (PDS) scheme, optical coupler-splitters are provided in nodes at the central office side and at midpositions in the transmission lines for the up-link optical signal from the subscribers to the central office as well as for the down-link optical signal from the central office to the subscribers. Even if such optical coupler-splitters exhibit the same wavelength characteristic when placed in the same environment, their wavelength characteristics change when they are disposed in different environments. In such a situation, the wavelength-multiplexing communication may sometimes be impossible; this problem becomes severer in higher density wavelength-multiplexing communications. The control function, which the optical transmitter of the subscriber is required to possess so as to overcome the problem, is not limited only for wavelength-stabilization of the light source but also wavelength control while monitoring variations in the wavelength characteristic of the network. This puts a heavy burden on the subscriber's terminal and hence inevitably raises its cost.